1. Field of the Invention
The present invention concerns apparatus for dispensing a sheet material and specifically to apparatus for automatically unwinding a sheet material in predetermined amounts at predetermined intervals. More specifically, the present invention concerns apparatus for dispensing sheet cleaning materials for cleaning solder paste stencils while the stencils are in use with a solder paste screen printer.
2. Background
The electronics industry has long made use of solder paste to hold surface mount and chip electronic components in relative orientation to lands on their respective printed circuit boards ("PCBs") prior to actually soldering the components in place. Solder paste screen printers apply solder paste to a printed circuit board ("PCB") in a predetermined pattern by utilizing a stencil having cutouts that replicate the land pattern of a PCB. During operation, the bottoms and cut-outs of such stencils become contaminated with solder paste and thus must periodically be cleaned. It is well known to equip solder paste screen printers with under-stencil wipe fixtures having thereon a roll of wiping material. The under-stencil wipe fixture is periodically traversed along the bottom side of the stencil to remove solder paste therefrom. The used wiping material is then advanced from a dispenser roll onto a take-up roll to expose a fresh, clean segment of wiping material prior to the next periodic pass. Such fixtures are typically configured with a full-width roll of wiping material, that is, the width of the wiping material is equal to that of the maximum width of the stencil which can be accommodated on a particular solder paste screen printer. Therefore, in situations in which the land pattern of the particular PCBs being printed is narrower than the maximum stencil width, a situation which occurs very frequently, a substantial amount of wiping material is wasted because it is used to wipe an unused segment or segments of the stencil. In addition to the waste of wiping material, because solder paste is considered to be hazardous, it is subject to onerous safe disposal requirements. That portion of the wiping material which is uncontaminated nonetheless adds to the bulk of the contaminated material and therefore needlessly increases disposal costs.
Related Art
U.S. Pat. No. 2,705,112 to Baumgartner, issued Mar. 29, 1955, discloses mounting material-dispensing rolls of varying widths onto a shaft at different longitudinal positions relative to one end thereof (column 1, lines 35 through 41). Specifically, Baumgartner discloses a stock roll supply arbor comprising a pair of removable core plugs that are dimensioned and configured to mate with the inner cylindrical core of the supply roll, one at each end thereof, to mount the supply roll onto a shaft. The shaft has a plurality of holes disposed along the length thereof and a pin is used to secure one of the core plugs to a selected one of the holes in the shaft.
U.S. Pat. No. 3,598,332 to Sharkey, issued Aug. 10, 1971, discloses a web supporting roller assembly comprising two removable end flanges (5) that are attached by screw means (7) to a tube (11) on which a roll of material is mounted, such that the edges of the material contact the inner lip of the flange at each end thereof.